1. Field of the Invention
The present invention is generally related to a photoresist for semiconductor processing and, more particularly, to a photoresist material which generates a positive or negative tone pattern depending upon the developing time and bake temperature.
2. Description of the Prior Art
A wide variety of wafer patterning techniques are currently in use in the semiconductor industry. Generally, patterning is a function of the resist chemistry, the patterning equipment, and the wafers themselves. It is well-known in the semiconductor art to vary the patterning by using either a positive or negative photoresist. Typically, a silicon crystal is placed in a diffusion furnace containing an atmosphere of oxygen. The surface of the silicon oxidizes to form a silicon dioxide (SiO.sub.2) overlay, which resists diffusion and, therefore, can be used to form a mask. The SiO.sub.2 is then coated with a photoresist and photographic etching processes are used to make the mask. The nature of the photoresist used to coat the SiO.sub.2 overlay determines the areas which are etched. The basis for all photoresists is a photolytic reaction in the film which changes the solubility of the exposed material. However, positive resists are completely different chemical systems from negative resists. Positive resists become soluble when exposed to UV radiation, leaving a protective film after the exposed portions of the film are dissolved. These resists preserve the tone of the mask and form a positive image of the mask. Conversely, in negative resists the exposed material becomes less soluble than the unexposed material. The wafer areas exposed to UV radiation are protected, thereby reversing the tone of the mask and yielding a negative image.